you posted this in the wrong "generation" part, but.. that doesn't really matter.
is the water/coolant mixture (50/50) correct? after the engine has been running for a few minutes, does the upper coolant tube (going to the top of the radiator from the engine) feel warm/hot (this indicates that, infact, the theromstat has opened. it could be the thermostat was installed backwards, which would not allow the coolant to flow out of the engine)?
with the engine cold, you can take the radiator cap off (the pressure cap) and run the engine to ensure there is flow in the radiator. (there will only be flow in the radiator after the thermostat has opened up, after a few minutes of running the engine)
going with a worst-case senerio, it could be the radiator is plugged, or the water pump has failed. of course, start with the simple things first: check for flow to the radiator.

After checking what he said, did you bleed the system? I normally pull the radiator cap and run them till there hot with the heater on full blast and keep filling it till the level in the radiator stops going down. Maybe you already knew that but I didnt see that anywhere. But air trapt in the system somewhere may cause overheating.

How badly is it overheating? Overheating is the number one cause for engine failure. It can cause alot of problems, such as a cracked block or a blown head gasket, so be careful. I would fix the problem before you start driving the truck again.